Add new option to WooCommerce to sort oldest to newest

woocommerce-modify-orderby.php

<?php
// Add "Sort by date: oldest to newest" to the menu
// We still need to add the functionality that actually does the sorting
// Original credit to Remi Corson: http://www.remicorson.com/woocommerce-sort-products-from-oldest-to-most-recent/
function patricks_woocommerce_catalog_orderby( $sortby ) {
	$sortby['oldest_to_recent'] = __( 'Sort by date: oldest to newest', 'woocommerce' );
	return $sortby;
}
add_filter( 'woocommerce_catalog_orderby', 'patricks_woocommerce_catalog_orderby', 20 );

// Add the ability to sort by oldest to newest
function patricks_woocommerce_get_catalog_ordering_args( $args ) {
	$orderby_value = isset( $_GET['orderby'] ) ? woocommerce_clean( $_GET['orderby'] ) : apply_filters( 'woocommerce_default_catalog_orderby', get_option( 'woocommerce_default_catalog_orderby' ) );

	if ( 'oldest_to_recent' == $orderby_value ) {
		$args['orderby'] = 'date';
		$args['order']   = 'ASC';
	}

	return $args;
}
add_filter( 'woocommerce_get_catalog_ordering_args', 'patricks_woocommerce_get_catalog_ordering_args', 20 );